Acid dyes were developed for application to the substrate from solution in a dye bath. The particular crystal form was not a matter of concern, since application was from solution and little was known about solubility behavior of different crystal forms. Attention to crystal form in the syntheses of acid and other water soluble dyes extended only to getting a form which could be separated from the reaction mixture with "reasonable" efficiency.
Conversely, the crystal forms of pigments have been thoroughly studied to achieve desired hues, maximum color yield and suitable vehicle emulsion stability. For example, see "The Chemistry and Physics of Organic Pigments", Lyde S. Pratt, John Wiley & Sons, Inc., New York, 1947. In recent years, the crystal form of "Disperse Dyes" has received much attention. This interest has generally been to achieve the crystal form stable to dye bath conditions from which they are applied to the fiber, e.g. U.S. Pat. Nos. 4,388,078; 4,374,640; 4,332,588; and references therein. Only one example of the modification of an acid dye has been reported, U.S. Pat. No. 4,474,577.